Most I am sure will say New Vegas. I can see where you are coming from. It is a great game.

I liked Fallout 3 personally. It was my first entry into the series. I was alive and gaming when the first 3 hit the market but wasn't on my radar. Then Fallout 3 came out. I bought it on launch day and spent more hours on that then I care to admit. It caused me to play the 1, 2 and tactics. It was what I wanted, RPG/FPS hybrid. If another comes out I will buy it as well. This is the only franchise I will buy a game from no matter what.

I've played New Vegas and the story is great, but for me, nothing is going to change that first time I realized I could blow up megaton and did it. You could play the game completely different from the start of the game. You can't sequence break Fallout 4, but you sure as hell can New Vegas and Fallout 3.

And i know you said no mods, but an upside to Bethesda running the show is if you dont like the game or get bored there is always a way to change it to your liking.

I think it was the same for me between the exaggerated and compressed world and terrain of Morrowind compared to the slightly more natural feeling Oblivion... iirc.. maybe i dont... all i remember is in both series one is ridiculously hilly and up and down and a little bland while the other is a bit flatter smoother and all around a little more natural feeling.

unmodded you are right New Vegas. Though the dlc giving weapons and armor at the start could ruin the challenge, and some of the DLC could also be game breaking for example lonesome road gave you access to a vendor with lots of money and repair very early game plus the repair option on ed-e making him massively more useful as a companion all for spending less then 10 minutes in the area then leaving.

Though to be fair 3 I haven't played in a long time as my prior PC would not let me get it running just crash at launch or black screen have not tried on the new computer, though I think at this point I would have to throw on a mod for iron sights as I was never much of a VATS user. I did greatly enjoy the DLC for this one just can't compare it as my memory is to far out of date to be fair to it.

4 I have played a good amount of I will never complain it was not worth the money. That said it has a few flaws for me some fixable by mods others not. (sorry for below I want to say this would be my favorite but these flaws combined stop it and some are more rant then me just mentioning a flaw)

The dialogue choices being little blurbs and having no idea what I'm about to say, yes mod fixable

The back story as a male you are ex military, as a female you are a trained lawyer and this will be referenced at points in the game but still not completely breaking the ability to roleplay,

This is my biggest issue and it is main story, you accept that in two (to you little) blackouts 300 years has passed but are 100% certain that the 300 years was the first and the second was a week maybe a few years at absolute most and you are determined you are looking for a baby or a child and adult is 100% not an option at any point, until you meet your son then oh yeah that makes sense let me just believe you on a concept that I have not even once considered as possible through the whole story.

This is an odd complaint your character is voice acted, for example so no matter how much you want to be insulting and dismissive of momma murphy telling you little to nothing in the start you come out being a winy sap in saying it. You can't do a build and feel like an ex-military soldier as you just have no intimidating sound to your voice and you know it since you can hear yourself. If you do play modded this means any expanded zones people make either they need to explain you being silent when others are voice acted or explain why your voice changed there but returns anytime you go back.

The leveling system. There is no cap and it is so easy to get a higher special score. This sounds good but as a result there is no reason to think about your long term build, long term every character would have every skill and every special at 10 The only build difference between completed characters is really if you min max them deciding what special you will take to 12 via the your special book and bobble heads instead of 11 (skill to 10 then drop it to 9 or lower use the book to push it up, then get the bobble head putting it up to 12, others just take to 10 then get the bobblehead to get 11). As a result faction is the only real difference between one character to another. The other games there were things you were good at and things you sucked at your character would have a personality they couldn't use a shotgun mini-gun sniper rifle and pistol interchangeable without suffering on some of them.

Weapon and armor choices are not really choices there are some that are just better, for example I can't come up with a reason to use the institute guns as the laser ones are better per shot and dps on auto plus I always tend to find one first. armor the only downside is weight bandit is crap protection and heavy so no reason to use it instead of metal or the next step of combat both better armor and combat being lighter as well synth is another step up and the only real downside being how stupid you look in it, any base clothing that doesn't allow armor parts is crap protection. for the head no helmet even competes with a trilby hat with ballistic weave. The marine stuff from far harbor is a waste as if you are that far in and going for max armor just go power armor so you don't go overweight the first time you pick up something while out. so for armor it is basically leather shadowed for a sneaky character, power armor, or combat/synth. so the flexible system they have is gimped by lack of real choices mods fix the armor side a bit by letting you wear armor pieces over everything and ballistic weave everything so you get more cosmetic base clothing options, but we are talking unmodded here

Eh. Gun variety is fine between the rifles in Nuka world and the 45-70 in Far Harbor. I'd like settlement interplay to be a little bit richer. But it's not that important to me so I usually just don't bother. I'll add things like beantown interiors, and some of the stuff that increases npc population & random encounters. But that's about it.